1. Field of the Invention
This invention relates to a character generator advantageously usable in printers such as serial-type thermal printers provided with a relatively inexpensive single array head. More in particular, the present invention relates to a character generator for thermal printers, which enables character imprints of excellent quality and uniform contrast to be produced by applying driving signals to the thermal printing head in accordance with the power application information previously stored in the character generator for each character pattern.
2. Description of the Prior Art
For relatively low-cost thermal printers, use is commonly made of a single array type thermal head which includes, for example, seven heat-producing elements arranged in a single array. Driving current signals are supplied to the heat-producing elements column by column from the data corresponding to a character pattern to be printed and the heat-producing elements are thereby selectively activated to produce heat for forming an imprint.
If the heat produced by each of the heat-producing elements fluctuates by deviating from a predetermined value, the printed characters will have uneven contrast and their quality will be deteriorated. Moreover, if an excessive heat is produced, the heat-producing elements may fail since they are usually made of an electrically resistive material and heat is produced by passing an electrical current through such an electrically resistive element. Thus, an excessive heat may cause deformation to such a resistive element to change its heat-producing characteristics, or the resistive element may be fused to cause disconnection in an extreme case. It should further be noted that the printing temperature of the heat-producing elements when activated is affected by changes in ambient temperature, fluctuations of a power supply, characteristic changes of the thermal head, the temperature increase of the thermal head as a whole due to a continuous, long-hour operation, etc.
In order to cope with the above-described problem, several proposals have been made to maintain the printing temperature of the heat-producing elements within a predetermined range, thereby allowing printed characters of uniform contrast to be obtained at all times.
First, it has been proposed to provide temperature detecting elements mounted on a thermal head to thereby control a driving power supplied to the heat-producing elements by the actual temperature of the heat-producing elements. It is true that such feed-back control systems will provide an accurate control of the temperature of the heat to be produced for printing operation; however, such systems tend to be expensive and require additional elements and complicated wiring. Thus, this approach also suffers from disadvantages and hinders the application to relatively inexpensive thermal printers.
A thermal head having a matrix of heat-producing elements arranged in, for example, 5 columns by 7 rows is known in the art. With such a thermal head having a matrix of heat-producing elements, it has been proposed to control the printing temperature by counting the number of dots to be activated in each column of a character pattern to be printed such that a large electric power is supplied to the columns having many dots to be activated and a smaller power is supplied to the columns having less dots to be activated. This approach allows the printing temperature of each of the heat-producing elements to be maintained in a desired range and a printed character without contrast variations may be obtained. It should, however, be noted that it requires the provision of a separate control circuit complicated in structure for counting the number of dots to be activated column by column in each character pattern. Thus, this approach also tends to be expensive and suffers from various disadvantages.
A further prior art printing temperature control system has been proposed in connection with single array type thermal printers. In accordance with this prior art technique, a driving power is supplied to the thermal head according to the arrangement of dots to be activated in each column of a character pattern to be printed. That is, a high driving power is supplied to those dots which are located at the head of a continuous chains of dots to be activated or which are located by themselves as isolated dots in a character pattern to be printed; on the other hand, a low driving power is supplied to the remaining dots in the continuous chains.
This third approach allows the printing temperature to be maintained within a preferred range. However, similarly with the above-described two cases, this technique is not free of disadvantages. For example, since the arrangement of the dots to be activated must be detected column by column in a character pattern to be printed in determining the condition to apply a driving signal for each of the dots in a column, it is required that a separate control circuit complicated in structure be provided, which then makes the whole apparatus bulky and expensive.